Enduracidin, a new antibiotic. II. Isolation and characterization

Structural diversity, biosynthesis, and biological functions of lipopeptides from Streptomyces

Covering: up to 2022Streptomyces are ubiquitous in terrestrial and marine environments, where they display a fascinating metabolic diversity. As a result, these bacteria are a prolific source of active natural products. One important class of these natural products is the nonribosomal lipopeptides, which have diverse biological activities and play important roles in the lifestyle of Streptomyces. The importance of this class is highlighted by the use of related antibiotics in the clinic, such as daptomycin (tradename Cubicin). By virtue of recent advances spanning chemistry and biology, significant progress has been made in biosynthetic studies on the lipopeptide antibiotics produced by Streptomyces. This review will serve as a comprehensive guide for researchers working in this multidisciplinary field, providing a summary of recent progress regarding the investigation of lipopeptides from Streptomyces. In particular, we highlight the structures, properties, biosynthetic mechanisms, chemical and chemoenzymatic synthesis, and biological functions of lipopeptides. In addition, the application of genome mining techniques to Streptomyces that have led to the discovery of many novel lipopeptides is discussed, further demonstrating the potential of lipopeptides from Streptomyces for future development in modern medicine.

Biosynthesis of Guanitoxin Enables Global Environmental Detection in Freshwater Cyanobacteria

Harmful cyanobacterial blooms (cyanoHABs) cause recurrent toxic events in global watersheds. Although public health agencies monitor the causal toxins of most cyanoHABs and scientists in the field continue developing precise detection and prediction tools, the potent anticholinesterase neurotoxin, guanitoxin, is not presently environmentally monitored. This is largely due to its incompatibility with widely employed analytical methods and instability in the environment, despite guanitoxin being among the most lethal cyanotoxins. Here, we describe the guanitoxin biosynthesis gene cluster and its rigorously characterized nine-step metabolic pathway from l-arginine in the cyanobacterium Sphaerospermopsis torques-reginae ITEP-024. Through environmental sequencing data sets, guanitoxin (gnt) biosynthetic genes are repeatedly detected and expressed in municipal freshwater bodies that have undergone past toxic events. Knowledge of the genetic basis of guanitoxin biosynthesis now allows for environmental, biosynthetic gene monitoring to establish the global scope of this neurotoxic organophosphate.

Supplemental Bacillus subtilis DSM 29784 and enzymes, alone or in combination, as alternatives for antibiotics to improve growth performance, digestive enzyme activity, anti-oxidative status, immune response and the intestinal barrier of broiler chickens

The present study investigated the effect of Bacillus subtilis DSM 29784 (Ba) and enzymes (xylanase and β-glucanases; Enz), alone or in combination (BE) as antibiotic replacements, on the growth performance, digestive enzyme activity, immune response and the intestinal barrier of broiler chickens. In total, 1200 1-d-old broilers were randomly assigned to five dietary treatments, each with six replicate pens of forty birds for 63 d as follows: (a) basal diet (control), supplemented with (b) 1 × 10⁹ colony-forming units (cfu)/kg Ba, (c) 300 mg/kg Enz, (d) 1 × 10⁹ cfu/kg Ba and 300 mg/kg Enz and (e) 250 mg/kg enramycin (ER). Ba, Enz and BE, similar to ER, decreased the feed conversion rate, maintained intestinal integrity with a higher villus height:crypt depth ratio and increased the numbers of goblet cells. The BE group exhibited higher expression of claudin-1 and mucin 2 than the other four groups. BE supplementation significantly increased the α-diversity and β-diversity of the intestinal microbiota and markedly enhanced lipase activity in the duodenal mucosa. Serum endotoxin was significantly decreased in the BE group. Compared with those in the control group, increased superoxide dismutase and glutathione peroxidase activities were observed in the jejunal mucosa of the Ba and BE groups, respectively. In conclusion, the results suggested that dietary treatment with Ba, Enz or BE has beneficial effects on growth performance and anti-oxidative capacity, and BE had better effects than Ba or Enz alone on digestive enzyme activity and the intestinal microbiota. Ba or Enz could be used as an alternative to antibiotics for broiler chickens.

Biosynthetic Pathways to Nonproteinogenic α-Amino Acids

Natural nonproteinogenic amino acids vastly outnumber the well-known 22 proteinogenic amino acids. Such amino acids are generated in specialized metabolic pathways. In these pathways, diverse biosynthetic transformations, ranging from isomerizations to the stereospecific functionalization of C-H bonds, are employed to generate structural diversity. The resulting nonproteinogenic amino acids can be integrated into more complex natural products. Here we review recently discovered biosynthetic routes to freestanding nonproteinogenic α-amino acids, with an emphasis on work reported between 2013 and mid-2019.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

As a versatile therapeutic modality, peptides attract much attention because of their great binding affinity, low toxicity, and the capability of targeting traditionally "undruggable" protein surfaces. However, the deficiency of cell permeability and metabolic stability always limits the success of in vitro bioactive peptides as drug candidates. Peptide macrocyclization is one of the most established strategies to overcome these limitations. Over the past decades, more than 40 cyclic peptide drugs have been clinically approved, the vast majority of which are derived from natural products. The de novo discovered cyclic peptides on the basis of rational design and in vitro evolution, have also enabled the binding with targets for which nature provides no solutions. The current review summarizes different classes of cyclic peptides with diverse biological activities, and presents an overview of various approaches to develop cyclic peptide-based drug candidates, drawing upon series of examples to illustrate each strategy.

Total Synthesis of (+)-Guadinomic Acid via Hydroxyl-Directed Guanidylation

Protecting-group-free total synthesis of (+)-guadinomic acid is reported using δ-valerolactone as a readily available starting material. The protocol utilizes the recent hydroxyl-directed guanidylation of unactivated alkenes as an approach for direct stereoselective incorporation of the guanidine unit furnishing the natural product in 7 steps.

Enduracididine, a rare amino acid component of peptide antibiotics: Natural products and synthesis

Rising resistance to current clinical antibacterial agents is an imminent threat to global public health and highlights the demand for new lead compounds for drug discovery. One such potential lead compound, the peptide antibiotic teixobactin, was recently isolated from an uncultured bacterial source, and demonstrates remarkably high potency against a wide range of resistant pathogens without apparent development of resistance. A rare amino acid residue component of teixobactin, enduracididine, is only known to occur in a small number of natural products that also possess promising antibiotic activity. This review highlights the presence of enduracididine in natural products, its biosynthesis together with a review of analogues of enduracididine. Reported synthetic approaches to the cyclic guanidine structure of enduracididine are discussed, illustrating the challenges encountered to date in the development of efficient synthetic routes to facilitate drug discovery efforts inspired by the discovery of teixobactin.

Total synthesis of teixobactin

To cope with the global bacterial multidrug resistance, scientific communities have devoted significant efforts to develop novel antibiotics, particularly those with new modes of actions. Teixobactin, recently isolated from uncultured bacteria, is considered as a promising first-in-class drug candidate for clinical development. Herein, we report its total synthesis by a highly convergent Ser ligation approach and this strategy allows us to prepare several analogues of the natural product.

Teixobactin is a recently identified antibiotic that shows activity against drug resistant strains of bacteria. Here, the authors report a highly convergent total synthesis of this natural product, with sufficient flexibility to also allow the synthesis of a number of analogues.

Cyclic lipodepsipeptides: a new class of antibacterial agents in the battle against resistant bacteria

In order to provide effective treatment options for infections caused by multidrug-resistant bacteria, innovative antibiotics are necessary, preferably with novel modes of action and/or belonging to novel classes of drugs. Naturally occurring cyclic lipodepsipeptides, which contain one or more ester bonds along with the amide bonds, have emerged as promising candidates for the development of new antibiotics. Some of these natural products are either already marketed or in advanced stages of clinical development. However, despite the progress in the development of new antibacterial agents, it is inevitable that resistant strains of bacteria will emerge in response to the widespread use of a particular antibiotic and limit its lifetime. Therefore, development of new antibiotics remains our most efficient way to counteract bacterial resistance.

Investigation on ultrasound-assisted extraction and separation of enduracidin from Streptomyces sp. NJWGY3665

Ultrasound-assisted extraction of enduracidin from Streptomyces sp. NJWGY3665 was studied. The effects of various factors on the yield of target components were investigated. The results showed that the extraction by ultrasound-assisted extraction is four times faster than those by conventional solvent extraction. The results also indicated that fast extraction rate was obtained in the first 30 min, and the maximum yield was obtained at the power of 150 W. The effects of other factors such as different solvents, solvent concentration, solvent to solid ratio, and extraction batches on the field were also discussed. The optimum conditions were found at solvent of methanol, time of 30 min, power of 150 W, pH of 7.0, solvent to solid ratio of 50 ml/g, solvent concentration of 70% and extraction batches of four times.

Enduracidin analogues with altered halogenation patterns produced by genetically engineered strains of Streptomyces fungicidicus

Enduracidins (1, 2) and ramoplanin (3) are structurally and functionally closely related lipodepsipeptide antibiotics. They are active against multi-drug-resistant Gram-positive pathogens, including MRSA. Each peptide contains one chlorinated non-proteinogenic amino acid residue, Cl(2)-Hpg or Cl-Hpg. To investigate the timing of halogenation and the importance of chlorination on bioactivity and bioavailability of enduracidin, and to probe the substrate specificity and portability of the ramoplanin halogenase, we constructed the mutant strain SfDelta30 in which the enduracidin halogenase gene orf30 had been deleted and complemented it with the ramoplanin counterpart orf20. We also expressed orf20 in the enduracidin wild-type producer. Metabolite analysis revealed SfDelta30 produced the novel analogues dideschloroenduracidins A (4) and B (5), while the recombinant strains SfDelta30R20 and SfR20 produced monodeschloroenduracidins A (6) and B (7) and a trichlorinated enduracidin (8), respectively. In addition, orf30 self-complementation yielded the strain SfDelta30E30, which is capable of producing six peptides including 6 and 7. MS/MS analysis positioned the single chlorine atom in 6 at Hpg(13) and localized the third chlorine atom in 8 to Hpg(11). Biological evaluation of these enduracidin analogues indicated that all retained activity against Staphylococcus aureus. Our findings lay the foundation for further utilization of enduracidin and ramoplanin halogenases in combinatorial biosynthesis.

Alanine scan of [L-Dap(2)]ramoplanin A2 aglycon: assessment of the importance of each residue

In efforts that define the importance of each residue and that identify key regions of the molecule, an alanine scan of the ramoplanin A2 aglycon, a potent antibiotic that inhibits bacterial cell wall biosynthesis, is detailed. As a consequence of both its increased stability (lactam vs lactone) and its "relative" ease of synthesis, the alanine scan was conducted on [Dap2]ramoplanin A2 aglycon, which possesses antimicrobial activity equal to or slightly more potent than that of ramoplanin A2 or its aglycon. Thus, 14 key analogues of the ramoplanin A2 aglycon, representing a scan of residues 3-13, 15, and 17, were prepared enlisting a convergent solution-phase total synthesis that consolidated the effort to a manageable level. The antimicrobial activity of the resulting library of analogues provides insight into the importance and potential role of each residue of this complex glycopeptide antibiotic.

Synthetic applications of aliphatic unsaturated α-H-α-amino acids

This article provides an overview of the literature concerning synthetic applications of unsaturated aliphatic amino acids in the period May 2000 to December 2004.

N-carbamoyl-2-(p-hydroxyphenyl)glycine from leaves of broad bean (Vicia faba L.)

1. dl-2-(p-Hydroxyphenyl)glycine was resolved through the bromocamphorsulphonate to give its d-isomer. The N-carbamoyl derivatives of these amino acids were synthesized. Circular-dichroism studies on these and related compounds, reported in a deposited Annex, helped to establish the optical configuration. 2. N-Carbamoyl-dl-2-(p-hydroxyphenyl)glycine was isolated from broad-bean leaves. It amounted to about 0.1% of the leaf dry matter. Racemization may or may not have occurred during the isolation. There were indications of the same compound in chicory and in savoy cabbage. Under weakly acidic conditions it was converted gradually into 5-(p-hydroxyphenyl)hydantoin. Both these compounds yielded 2-(p-hydroxyphenyl)glycine on acid hydrolysis. 3. The occurrence is discussed of 2-phenylglycine derivatives in Nature and of N-carbamoyl-amino acids and hydantoins in plants. 4. Gradient elution from anion-exchange resin with acetic acid, besides proving useful for the present work, gave useful separations of pyrrolidonecarboxylic acid and of some N-acetyl-amino acids. 5. Supplementary material (Annex 1: details of experimental work other than ultraviolet and circular-dichroism spectra; Annex 2: ultraviolet absorption and circular dichroism of d-2-phenylglycine and some related compounds) has been deposited as Supplementary Publication SUP 50003 at the National Lending Library for Science and Technology, Boston Spa, Yorks. LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1971), 121, 7.